Sensors comprising quartz crystal microbalance (QCM) resonators are generally known. QCM resonators may function as acoustic wave resonators to provide highly sensitive detection mechanism for fluid analytes. As illustrated in FIG. 1, a typical QCM resonator is shown generally at 100 and comprises a piezoelectric crystal substrate 102 located between a pair of electrodes 104 having leads 106. In this configuration, an electric field may be generated by the electrodes 104 and extend therebetween along a transverse axis, or through the thickness, of the piezoelectric crystal substrate 102. Hence in this configuration the QCM resonator may be termed a thickness field excitation (TFE) resonator. The electrodes 104 and the crystal 102 are dimensioned to achieve an optimal resonance condition.
One particular example of a TFE resonator is described in U.S. Pat. No. 6,544,478 to Oyama et al wherein the resonator is arranged in a multi-channel structure. The resonator includes a crystal substrate that has four mutually opposed electrodes disposed on opposite sides of the substrate. In operation, the TFE resonator may be used to detect and quantitatively analyze components of a sample from a variation in fundamental resonant frequency and impedance when a surface of one of the pair of electrodes is immersed into either a sample gas or solution.
While the above TFE resonators have been suitable for use with non-caustic analytes, it has been found that when these resonators are immersed into a caustic substance the electrodes tend to deteriorate as a result of contact between the electrodes and a caustic, corrosive, or electrode-degrading analyte, which oftentimes causes electrode deterioration, reduced sensitivity, and inaccurate sensor readings. When QCM electrodes are made with a gold layer on top of a chrome or titanium adhesion layer on the surface of quartz, corrosive analytes such as a strong acid will attack the adhesion layer when contacting the electrode. For example, aqua regia, which is a mixture of nitric acid and hydrochloric acid, will even dissolve gold, resulting in lifting the prior art electrode off of the adhesion layer. Also, use of these resonators is restricted to non-conductive analytes because of the possibility that the electric field may become shorted. Accordingly, to date, no suitable QCM resonator is available for analyzing a caustic, corrosive, or conductive analyte. Thus, there has been a long-felt need for resonators configured with electrodes that are not damaged or deteriorated because of contact with caustic, corrosive, or conductive analytes.